From everything I've got so far from reading and talking to everybody (people who actually own and know), is that 7.62 would be the choice. Here's what I've found:

1) If you don't want or can't afford multiple cans, you'll get more use out of a 7.62 since it's works on a 308 (should you ever get different caliber rifles)
2) The sound difference is not noticable... some even say it's less sharp... not quieter, just different
3) More durable for a 5.56 than a 5.56 can

Now, again, I'm no expert... I do not own a can yet... I"m still waiting for my stamp on my SBR. But this is what I've come up with after a lot of hunting around for info. I welcome anyone to correct me, however. If I'm wrong about these things, I want to know it as well as help better define the answers and parameters we're all always searching for.